A typical induction log is equipped with coils which are mounted at specified locations on a sonde and which form current flows in formations adjacent to a well borehole. Typically, they are made to investigate a particular radial depth extending outwardly from the well borehole. One set of coils is constructed for what is normally denoted as the medium depth investigation and the other set is for the deep radial investigation. Normally, the medium and deep distances are fixed as a matter of design factors relating to the deployment of the coils on the sonde. Many logs are evaluated on the basis of the data obtained from the medium and deep induction logs. Sometimes, it is especially important to have two depths of measurement because the resistivity will vary in relation to the depth of fluid infiltration from the borehole. So to speak, the drilling fluid filtrate flows into the adjacent formations to a depth dependent on the nature of the formation, the nature of the drilling fluid and other factors so that there is a variation in formation resistivity as a result of fluid infiltration. Since this depth of infiltration varies, it may be helpful to change the depth of investigation so that the actual depths of fluid infiltration can be determined. There are other reasons why changes in the depth of investigation may be desirable. Therefore, the present disclosure sets forth a variable depth investigation for an induction log system and one which provides the depth of investigation in an adjustable or variable fashion.
To accomplish this in the past, additional receiver coils have been installed at intermediate spacings. This has enabled an extension of the depth of investigation to a greater radial distance from the borehole. This can be accomplished at a cost which relates to the complexity of the enhanced coil system, the multiplicity of coils involved, and handling of the data from the various coils. By contrast, the present disclosure sets forth an approach which synthesizes an induction log system which provides investigation at a plurality of depths utilizing the two basic logs which are well known, namely the one capable of a medium radial depth, and the other providing a deep radial investigation.
This arrangement utilizes data from the well established medium and deep investigation induction log system. Moreover, this system is constructed and arranged so that a different radial depth of investigation can be accomplished without structurally changing the construction of the coils deployed on a sonde i.e., no changes are required from equipment in existence presently. The present approach utilizes the two popular depths of investigation to provide a plurality of radial depths by combining the two logs from known but different radial depths of investigation in different ways so that a modified radius can be obtained. Consequently, the present disclosure is summarized as a system which provides a linear conductivity signal which reflects a different radius of investigation, and is further able to accomplish this without reconstruction of the coils deployed on a sonde. Moreover, it involves delayed processing of the two signals (one from the deep radial investigation and one from the medium radial investigation) and thereby enables modification of data lone after the log has actually been made.